Psychrophilic (6-15 degrees C) anaerobic treatment of malting wastewater was investigated. A two-module expanded granular sludge bed reactor system with a total volume of 140 dm(3) was used to treat malting wastewater having a soluble and total chemical oxygen demand (COD) between 233 and 1778 mg dm(-3) and between 317 and 4422 mg dm(-3), respectively. The removal efficiencies at 6 degrees C were 47 and 71% of the soluble and volatile fatty acids (VFA) COD, at organic loading rates (OLR) ranging between 3.3 and 5.8 kg of COD m(-3) day(-1). The removal efficiencies at 10-15 degrees C were 67-78 and 90-96% of the soluble and VFA COD at an OLR between 2.8 and 12.3 kg of COD m-3 day-l. The specific methanogenic activity of the sludge present in each module increased 2-3-fold during system operation for 400 days. The relatively high concentration of suspended solids in the influent (25% of the total GOD) caused a deterioration of the sludge bed in the first reactor module. This was aggravated by excessive growth of acidifying biomass, which persisted in the first module sludge bed and resulted in granular sludge flotation. However, the second module could accommodate the increased OLR, thus providing a very high effluent quality (soluble COD < 200 mg dm(-3)) of the total system. The stability of module I concerning suspended solids could be restored by presettling the wastewater.